Axial flow fan structure



AXIAL FLOW FAN STRUCTURE Filed Aug. 4, 1941 4 Sheets-Sheet l 1,? Fig. i a

J71 eador/Y. Waller P 1942- T. H. TROLLER 2,294,586

AXIAL FLOW FAN STRUCTURE Filed Aug. 4, 1941 4 Sheets-Sheet 2 40 4 4 1, b m60d0f' H I OllBl Sept. 1, 1942. T. H. TROLLER AXIAL FLOW FAN STRUCTURE Filed Aug 4, 1941 4 Sheets-Sheet 3 Patented Sept. 1, 1942 AXIAL FLOW FAN STRUCTURE Theodor H. Troller, Akron, Ohio, assignor to La- Del Conveyor .9; Manufacturing Company, New Philadelphia, Ohio, a corporation of Ohio Application Augusta, 1941, Serial No. 405,323

11 Claims. (01. 230-117) The invention relates to propeller type fans for moving air or fluids under pressure through ducts, and more particularly to such fans wherein the fan motor is located within the duct.

There are many advantages derived from cating the fan motor in the air duct of a propellor type fan, including the fact that there is an inner fairing with a long converging tail portion provided coaxial with the fan in order to occupy the space behind the fan where pressure is not properly built up thereby, and the fan motor can conveniently be located within said inner fairing and the motor shaft directly connected to the fan.

However, when the motor is located within the inner fairing a serious problem of cooling the motor arises, because the inner fairing should constitute a streamlined substantially continuous shell in order to offer the least possible resistance to the air flow through the duct, and the motor is accordingly substantially completely enclosed by the inner fairing.

In certain prior constructions it has been proposed to cool the motor by conducting excessive heat therefrom through the metal frame of the motor and the metal fairing into the airstream. This method of cooling is not at all satisfactory for large motors or motors of any substantial size because the heat conducting surface is not large enough as compared with the size of the motor to conduct sufficient heat to cool the motor.

On the other hand, even where a small motor is used it is essential to have good metal-tometal contact from the motor frame to the fairing to insure good heat conduction for cooling the motor. Due to the fact that with propeller type fans operating in ducts, the diameter of the inner fairing is substantially fixed by the required pressure build up in the duct and the motor speed, the result is that a small motor frame may be much smaller than the required inner fairing and in such case the motor frame must therefore be built up considerably to mount it in the fairing. This condition makes it increasingly difficult to obtain the necessary metal-tometal contact for satisfactorily conducting heat away from the motor.

. In certain installations, it is feasible to bring cooling air to the motor from outside the air duct, by conducting it through streamlined conduits in the air stream and through the inner fairing to the motor. However, this is a complicated and expensive procedure involving a certain amount of exterior space which very frequently is not available.

It has been proposed to cool the fan motor by exhausting the hot air from the motor out through ports in the nose of the inner fairing ahead of the fan. There are two serious disadvantages to this method: first, the exhausting hot air interferes with and reduces the eillciency of the air flow through the duct, and second, the exhausted hot air is recirculated through the motor, which minimizes the cooling effect.

It is therefore a principal object of the present invention to provide an improved axial flow fan construction having the fan motor located within the air duct, said construction embodying means and methods for utilizing air flowing through the duct for cooling the motor, and at the same time increasing the eillciency of the fan or blower.

More specifically, it is an object of the invention to provide novel means for cooling the motor by utilizing a portion of the impelled air which has lost some of its energy, in such a way that uniformity of velocity distribution is increased behind the fan.

Another object is to provide improved means for circulating a part of the impelled air through and around the motor and exhausting it into the air stream in such a way as to still further improve uniformity of velocity distribution behind the fan.

A further object is to provide novel means for air cooling the motor and increasing air flow eillciency, which means is adapted for use with varying sizes and various types of motors located within the inner fairing.

Another object is to provide novel means for cooling the fan motor and at the same time permitting a reduction in length of the converging tail portion of the inner fairing.

A still further object is to provide means and methods for air cooling the motor and increasing air flow efliciency, which can be applied to existing axial flow fan structures with a minimum of time, labor and expense.

These objects and others which may be apparent from the following description, are accomplished by the improvements comprising the present invention, which may be briefly stated in general terms as comprising providing air intake ports in the inner fairing and behind the propeller and conducting a part of the impelled air through the ports to and around the motor for cooling the same, said cooling air being then directed, preferably by an auxiliary blower within the fairing, out through the converging tail of the fairing axially thereof, where it acts to aid in producing uniform velocity distribution in the air stream behind said inner fairing.

Referring to the drawings in which preferred embodiments of the invention are shown by way of example,

,Figure 1 is a longitudinal sectional view of a fan and duct construction embodying the present invention, and showing one type of motor located within the inner fairing in the duct;

Fig. 2 is a fragmentary transverse sectional view taken on line 22, Fig. 1;

Fig. 3 is a fragmentary transverse sectional view taken on line 33, Fig. 1;

Fig. 4 is a longitudinal sectional view similar to Fig. 1, showing another type of motor located within the inner fairing;

Fig. 5 is a fragmentary transverse sectional view as on line 5-5, Fig. 4;

Fig. 6 is a fragmentary longitudinal sectional view similar to Fig. 4 showing modified intake ports;

Fig. 7 is a longitudinal sectional view similar to Fig. 1, showing still another type of motor located in the inner fairing;

Fig. 8 is a fragmentary transverse sectional view as on line 8-8, Fig. 7;

Fig. 9 is a longitudinal sectional view similar to Fig. 4, showing a modified arrangement of air intake ports through the fairing;

Fig. 10 is a longitudinal sectional view similar to Fig. 1, showing a further modified arrangement of intake ports in the converging tail; and

Fig. 11 is a fragmentary elevation of the converging tail portion of the inner fairing, showing a still further modification of air intake ports.

Similar numerals refer to similar parts throughout the several views of the drawings.

The fan construction shown and described herein is particularly adapted for use in ventilating systems where it is desired to convey air against substantial pressure through a duct. The entrance end of the duct is indicated at I and may be outwardly flared as shown, and the portion of the duct surrounding the propeller blades, straightener vanes and the tail portion of the inner fairing is indicated at 8, bein adapted for connection at its exhaust end 9 with a suitable duct for conveying air to desired locations.

The inner fairing indicated generally at H! is coaxial with the outer duct 8 and is supported therein by a series of circumferentially arranged straightener vanes II which are secured at their inner ends to the cylindric portion ID of the inner fairing H! and at their outer ends to the outer duct 8. These straightenervanes are located behind the blades |2 of the fan and serve to straighten out the air flow while avoiding losses in energy of the air stream connected with its reduction in velocity while passing through the straightener vanes. Preferably the straightener vanes II are designed and constructed substantially in accordance with my prior Patents No. 2,219,499, dated October 29, 1940, and entitled Propeller type fan construction, and No. 2,040,- 452, dated May 12, 1936, and entitled Fan construction.

The hub l3 of the fan is preferably secured on the shaft M of the motor l5 mounted within the inner fairing l0, and the outer surface l6 of the hub at its rear end is made equal in diameter to the diameter of the cylindric portion ID of the fairing so as to provide a minimum of resistance to the air stream, a rounded nose l1 being secured to the hub I6 for the same purpose. The

though this number can be varied as desired, and the blades are preferably of substantially standard design having a greater pitch at the hub than at their outer ends.

The inner fairing I0 is provided with a converging tail portion l8 at its downstream end, and the tail portion l8 begins just behind the straightener vanes H and converges gradually therefrom to a relatively small circumference at its rear end IS. The front end of the tail portion may be rabbeted as indicated at 20 to make a smooth exterior connection 'with the straight cylindric portion I0 of the inner fairing, and may be connected thereto by means of screws 20' screwed through the tail portion and into a ring flange on the straight cylindric portion l0.

As shown in .Fig. 1, the motor 5 is considerably smaller in circumference than the inner surface of the fairing l0, and an enclosure is provided for the motor within the fairing II). This enclosure may include tubular members 2| encirblades l2 may be four in number as shown, al-

cling the end portions of the motor. Spacer supports 22 are secured at circumferential intervals between the motor end portions and the tubular members 2| for non-rotatably mounting the motor IS. The tubular members 2| are thus spaced radially outward from the motor, and at their.

inner or adjacent edges they are connected to a cylindrical housing member 23 by reinforcing rings 24 surrounding the central portion of the motor. The cylindrical housing member 23 has outwardly projecting spacer bars 25 secured thereto at circumferential intervals, and the cylindrical portion of the inner fairing is preferably secured to the spacer bars 25 by means of screws 26.

As shown, the front member 2| has secured thereto a cup-shaped end piece 21 spaced from the front end of the motor and having an axial intake aperture 28 therein through which the motor shaft I4 extends forming an annular opening. A cup-shaped bafile shield 21 extends from the front end of portion In of the inner fairing forwardly and inwardly to form an air passage in front of end piece 21. An auxiliary fan or blower 29 is mounted on the motor shaft I4 between the front end of the motor and the end wall of the end piece 21, for sucking air through the intake opening 28 and forcing it rearwardly around the outside of the motor proper and within the enclosure consisting of members 2| and 23 surrounding the motor. A dome-shaped deflector end member 30 may be secured to the rear housin member 2|, and is provided with an axial aperture 3| for exhausting the air which is forced around the motor by the fan 29.

The means for supplying cooling air to the motor from the air stream impelled by the propeller fan |2 preferably includes a series of intake ports 32 located in the converging tail portion |8 of the inner fairing. As shown in Figs. 1 and 3 the ports 32 may be rectangular and may comprise a circumferential series of four ports located at the start of the converging portion.

Baffle means for directing air entering the intake ports 32 forwardly between the sectional housing around the motor "and the inner fairing may consist of a ring 33 of sheet metal and the like having its rear edge secured to the inner surface of the tail portion 8 immediately adjacent to the ports 32. and having its front end flanged and secured to the outer periphery of the dome-shaped end member 38 of the sectional housing. I

When the propeller fan is operated by turning on the motor IS, the air is delivered by the blades I! through the straightener vanes II toward the right as viewed in Fig. 1. As this air travels between the outer duct 8 and the inner fairing,

. friction with the inner fairing slows up a thin inner layer of air. It is well-known that the rear end or the tail portion of the inner fairing is streamlined with a gradual convergency to permit the transition of velocity energy of the air stream to static energy without undue losses in total energy, and as the converging portion of the tail is traversed by the air stream the inner layer of air which is losing energy tends to leave the flared portion and cause vortices of air, which decrease the uniformity of velocity of the air stream and accordingly decrease efficiency of the same.

By providing the intake ports 32 for supplying cooling air to the motor and locating them in the converging tail portion, some of the inner layer of air which has lost much of its energy due to friction is removed, and an outer layer of fully energized air moves inwardly and is substituted along the inner fairing. This substituted inner layer of air has suflicient energy to overcome the adverse or rising pressure gradient beginning at the start of the tail portion and ending at its converging end. The result is that the velocity distribution in the duct at the converging end of the tail portion is greatly improved, because there is very little air which has lost its energy present at the converging end of the tail portion and at the axis of the duct.

As shown by the arrows in Fig. 1, air from the inner layer along the outer surface of the tail portion I8 is drawn through the ports 32 forwardly between the enclosure for the motor and the inner fairing and through the opening 28 in the end member 21, by the fan 29. From the fan 29 the air is forced along and around the motor I within the enclosure and exhausted into the converging tail portion through the axial exhaust opening 3| from which it is exhausted directly into the air stream through the axial opening 34 in the rear end I9 of the tail portion l8.

By drawing in to the motor the inner layer of impelled air which has lost much of its energy and circulating this air through and around the motor by means of the fan 29, a substantial amount of energy is restored to this air so that when it is exhausted through the axial opening 34 of the tail portion into the air stream, air having an axial velocity is being supplied at the point where it is normally hardest to get velocity due to the frictional losses along the inner fairing and its converging tail. Consequently, the exhausting of the motor cooling air through the opening 34 greatly aids in producing an even velocity distribution in the air stream behind the inner fairing.

Moreover, the substantial prevention of vortices of air along the tail portion by drawing the inner layer of air through the fairing, results in a reduction in the required length of the tail portion I8 as compared with usual practice, because the transition from velocity energy to static energy is made possible more abruptly.

In Figs. 4 and 5 the construction of the outer duct to, the inner fairing Illa, and the straightener vanes Ila is substantially identical with that shown in Figs. 1, 2 and 3. The propeller hub I3a, the propeller blades 12a and the rounded nose Ila are also substantially identical to the construction shown in Figs. 1, 2 and 3.

The motor Iia which is located within the inner fairing Illa is a different conventional type of motor from that shown in Figs. 1, 2 and 3, and has the usual open frame at the ends through which air can circulate for cooling the motor. Accordingly, it is not necessary to provide a separate enclosure for the motor within the inner fairing, and the auxiliary fan for circulating the cooling air can conveniently be located in the converging tail portion.

As shown the auxiliary fan 29a is secured on the motor shaft Ila at the rear end thereof and is located close to the converging end of the tail portion I8a. The intake ports 32a for the cooling air are located in the front end of the tail portion I8a, and a tubular baflie member 35a is secured to the inner surface of the tail portion I 8a immediately behind the ports 32a and extends therefrom to the central portion of the motor I5a as shown. The motor is supported within the fairing Illa by means of spacer bars 25a projecting outwardly from the motor and secured to the straight cylindric portion IOa of the inner fairing by means of screws 26a. Spacer supports 22a extend between the inner fairing cylindric portion 1'11 and the front end of the motor frame, for preventing movement of the motor in the inner fairing.

As shown by the arrows in Fig. 4, as the auxiliary fan 29a sucks in the inner layer of cooling air from along the outer surface of the converging tail portion Ila, the air travels forwardly between the fairing and the motor frame and then in to the motor at its front end and out through its rear end to be exhausted through the axial opening 34a in the tail portion. Thus the same advantages, with respect to cooling the motor and improving the air flow at the tail portion and increasing the uniformity of velocity distribution, are attained with the different type of motor I5a and the slightly modified construction within the inner fairing shown in Figs. 4 and 5.

The holes or intake ports 32 and 32a in the tail portion of the inner fairing should be so shaped as to make an efllcient transition from velocity energy to static energy in the cooling air which is conducted to the motor. In some cases lips or scoop-shaped protuberances on the tail over the intake ports will help to make such transition. As shown in Fig. 6 the tail portion I8b of the inner fairing Illb surrounding the motor I5b is provided with lips 40b which are substantially arcuate in cross section, and which project outwardly and forwardly from the rear ends of the intake ports 32b and substantially cover the same. Obviously, these lips "b serve to scoop a portion of the inner layer of impelled air into the inner fairing through the ports 32b.

The total cross sectional area of the openings formed between the front ends lb of the lips and the outer surface of the inner fairing is cal culated to be sufficient for taking in the required amount of cooling air for cooling the motor and for improving the velocity distribution in the air stream behind the inner fairing. In many cases it has been found that the total cross sectional area of the openings formed by the lips may be as low as 1 to 5 per cent of the cross sectional area of the air flow in the duct at those points, and still function to take in a required amount of cooling air efficiently and control the air flow to increase velocity distribution and improve the efliciency of the fan.

In Figs. 7 and 8 the construction of the outer duct 8c, the inner fairing 1c, and the straightener vanes I Ic is substantially identical with that shown in Figs. 1, 2 and 3. The propeller blades I2c are shown mounted on a hub indicated at I3c having means for adjusting the pitch of the blades, and a rounded nose is secured on the front end of said hub.

The motor Iic which is located within the inner fairing IIlc is a conventional motor of a different type from motors I and I5a shown in Figs. 1 and 4 respectively, and the motor I5c has an auxiliary fan or blower 29c mounted on the motor shaft I40 and located in the rear end of the motor between the cup-shaped end housing piece 42c and the motor frame 430. Air for cooling the motor I5c is taken in axially through the rear end of the motor to the intake of auxiliary fan 290 which circulates the air forwardly between the motor frame and the inclosure 44c around the same.

The means for supplying cooling air to the motor from the inner layer of air impelled over the fairing Inc by the propeller fan blades I20, preferably includes a circumferential series of intake ports 320, which may be four in number and located near the forward end of the converging tail portion I8c.

Conduit means for conveying air from said ports 320 to the intake of auxiliary fan 290 may include a tubular intake spider 450 having its central hub communicating with the rear end of the motor and having four radially projecting tubular arms 46c. Preferably extension tubes 410 are telescopically fitted in said arms 460 for insertion in the ports 320, to facilitate assembly of said spider 450 in said tail I80.

A baffle is preferably provided at the front end of motor I50, for deflecting air which has passed over the motor and directing it rearwardly. As shown, a cup-shaped bafile 480 is secured to the front end of the motor and has an annular flange 490 for deflecting air flowing forwardly between the motor frame 430 and enclosure 44c, and directing it rearwardly through the passage provided between enclosure 44c and fairing I00 and around the circumferentially arranged spacer supports 25c.

As indicated by the arrows in Fig. 7, the motor cooling air flows rearwardly around and past the tubular arms 46c and 410 of the intake spider 45c, and is exhausted axially through the opening 340 in the rear end of the converging tail I8c. Thus the same advantages, with respect to cooling the motor and improving uniformity of velocity distribution at the rear end of the tail, are attained with a motor of the type of I50 and the construction of Figs. 7 and 8.

It will be understood that other types of conventional motors can be used and the purposes of this invention attained by slight modifications in the motor supports, air ducts and baiiles within the inner fairing, without departing from the scope of this invention as defined in the claims.

Referring now to Fig. 9, the motor I5d is of the type shown in Fig. 4, and is located within a Streamlined inner fairing Ind within a duct 8d, and there being a propeller fan with blades l2d and a rounded nose IId located at the front end of said inner fairing and straightener vanes Ild behind the propeller fan and extending between said fairing and said duct.

The air intake through the fairing Ind for supplying cooling air to the motor in this embodiment of the invention is located immediately bebind the propeller blades I211, and preferably consists of a circumferential slot 5041 through the fairing between the rear end of the propeller hub IM and the straightener vanes IId, or in other words, between the rotating part and the stationary part of the inner fairing.

The auxiliary fan 29d within the fairing Ilia is secured on the motor shaft Ild at the rear end of the motor and is located close to the exhaust opening 31d in the tail IBd. Preferably a baflie ring ild is secured to the front ends of the spacer supports 25d for closing off the annular space between the motor of the fairing.

In the air delivered by the blades I2d of the propeller fan, the thin inner layer flowing over the inner fairing which loses energy due to friction begins immediately behind the propeller blades and increases somewhat in thickness as it flows over the remainder of the fairing. By

. supplemental to intake ports in the tail I8d, such as shown in Fig. 4, but in many cases the effectiveness of the slot 50d will be suflicient for accomplishing the purposes of the present invention, so that the sides of the tail may be completely closed as shown in Fig. 9.

Referring to Fig. 10, the motor I5e is of the type shown in Fig. 1 and the supporting construction within the fairing Ille is generally similar to that shown in Fig. 1, except that a conically shaped baflie ring 33c extends from the rear end of the motor rearwardly into the converging tail I8e and is secured thereto adjacent to the exhaust po t 34c. 4

This particular extended shape of baflie 33a is required because in the converged tail I8e shown in Fig. 10, two longitudinally spaced circumferential series of intake ports Me and 32'e are provided, the rear ports 32's being preferably staggered with respect to ports 32c. Because of having two series of intake ports sucking air from the inner layer of impelled air flowing over the outer surface of the converging tail, it is possible to furthershorten the length of the tail and still get an efficient transition from velocity energy to static energy. Accordingly, it will be observed that the tail I8e is shorter than the tails I8, I81: and I8c having one series of intake port s.

Referring to Fig. 11, another modified construction of the converging tail is shown at I81 connected to an inner fairing I0). In this embodiment, instead of providing substantially rectangular air intake ports through the tail, elongated relatively narrow arcuate or circumferential slots 52f are provided, being closely spaced circumferentially by narrow portions 53f of the tail.

All of the embodiments of the invention herein shown and described provide an axial flow fan structure having the fan motor located within a fairing in a duct, said structure embodying means and methods utilizing air flowing through the duct for cooling the motor and simultaneously increasing the efliciency of the fan.

I claim:

1. Axial flow fan structure including an outer duct, a streamlined inner fairing having a converging tail at its rear end, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said propeller fan, a series of straightener vanes of air flowing along the outer surface of the fairing, and an auxiliary fan within said fairing driven by the motor for circulating air from the intake opening through the motor and exhausting it through the end of said converging tail.

2. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said fan, a series of straightener vanes secured to said duct and said inner fairing immediately behind said fan, said inner fairing having a converging tail portion immediately behind said straightener vanes, said converging tail portion having air intake ports adapted for scooping air from the inner layer of impelled air flowing along the surface of said fairing and conducting it into said fairing for cooling the motor, and auxiliary fan means within said fairing driven by said motor for exhausting said air through the converging end of said tail portion axially of said duct.

3. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said fan, said inner fairing having a converging tail portion at its rear end, said converging tail portion having air intake ports therein, auxiliary fan means within said fairing driven by said motor for sucking air through said intake ports, and baffle means in said fairing for circulating said air through the motor and then exhausting it through the converging end of said tail portion axially of said duct.

4. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said fan, a series of straightener vanes secured to said duct and said inner fairing immediately behind said fan, said inner fairing having a converging tail portion immediately behind said straightener vanes, said converging tail portion having a series of circumferentially arranged air intake ports at its front end immediately behind the rear ends of said straightener vanes, said ports being adapted for withdrawing air into said fairing from the inner layer of air flowing along the outer surface of the fairing, and an auxiliary fan within said fairing driven by the motor for circulating air from the intake ports through the motor and exhausting it through the rear end of said tail portion.

5. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said fan, a series of straightener vanes secured to said duct and said inner fairing immediately behind said fan, said inner fairing having a converging tail portion immediately behind said straightener vanes, said converging tail portion having longitudinally spaced circumferentially arranged series of air intake ports, said ports being adapted for withdrawing air into said fairing from the inner layer of air flowing along the outer surface of the fairing, and an auxiliary fan within said fairing driven by the motor for circulating air from the intake ports through the motor and exhausting it through the rear end of said tail portion.

6. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said fan, said inner fairing having a converging tail portion at its rear end, said converging tail portion having air intake ports adapted for withdrawing air into the fairing from the inner layer of air flowing along the outer surface of said fairing, lips projecting outwardly and forwardly from the rear ends of said intake ports for scooping air into said ports, and auxiliary fan means within the fairing and driven by said motor for circulating air from said ports through the motor and exhausting it through the rear end of said tail portion.

7. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said propeller fan, an auxiliary fan mounted in the rear end of said motor and driven thereby, said inner fairing having a converging tail portion at its rear end, said converging tail portion having air intake ports for withdrawing air into the fairing from the inner layer of air flowing alongthe outer surface of said fairing, tubes for conducting said air radially inward from said intake ports to the intake of said auxiliary fan for being circulated forwardly through the motor, and baffle means for deflecting said air rearwardly past said tubes to be exhausted through the rear end of said tail portion.

8. Axial flow fan structure including an outer duct, a streamlined inner fairing, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said propeller fan, an auxiliary fan mounted in the rear end of said motor and driven thereby, said inner fairing having a converging tail portion at its rear end, said converging tail portion having an axial exhaust opening and side air intake ports for withdrawing air into the fairing from the inner layer of air flowing along the outer surface of said fairing, walls in said fairing providing passages conducting said air radially inward to the intake of said auxiliary fan for being circulated forwardly through the motor and rearwardly past said passages to the axial exhaust opening in the end of the tail portion.

9. Axial flow fan structure including an outer duct, a streamlined inner fairing having a converging tail provided with an axial exhaust port in its rear end, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said propeller fan, said inner fairing being provided immediately behind said propeller fan with a circumferential air intake slot adapted for withdrawing air into the fairing from the inner layer of air impelled along the outer surface of the fairing by the propeller fan, and an auxiliary fan within said fairing driven by the motor for circulating air from said intake slot through the motor and exhausting it through the axial exhaust port in said converging tail.

10. Axial flow fan structure including an outer duct, a streamlined inner fairing having a converging tail at its rear end, a propeller fan rotatably mounted in said duct coaxial with said inner fairing, a motor in said fairing for driving said propeller fan, a series of straightener vanes within said duct immediately behind said propeller fan, said inner fairing being provided hehind the propeller fan with an air intake opening adapted for withdrawing air into said fairing from the inner layer of air flowing along the outer surface of the fairing, and an auxiliary fan within said fairing driven by the motor for circulating air from the intake opening around the motor and exhausting it through the end of said converging tail.

11. Axial flow fan structure including an outer duct, a streamlined inner fairing having a converging tail at its rear end, a propeller fan rotatably mounted in said duct coaxial with said 

